Vertical and seasonal variation of phytoplankton photosynthesis in a brown‐water lake with winter ice cover

Abstract

SUMMARY. Unlike previously studied lakes with prolonged winter ice and snow cover, Lake Paajarvi, southern Finland, has a high humus content and consequently differs in both the quantity and quality of light penetration into its waters. Moreover, the range of temperature fluctuation and the degree of development of thermal stratification are greater in Paajarvi, and this increased environmental heterogeneity apparently stimulates diversity in the phytoplankton community, especially in the seasonal succession of species. Differences in the photosynthetic capacity of algae from different depths in the water column were not great. This is attributed to the extremely shallow euphotic zone, algae circulating freely through the steep light gradient and sedimenting rapidly once they pass through the thermocline into the hypolimnion. It is suggested that ‘adaptation’ of phytoplankton to the great seasonal changes in irradiance is achieved largely by successive growths of different species in the community, and that the adaptations and vertical migrations by individual algal species, which have been reported from polar and high alpine lakes, may be of secondary importance in Pääjärvi. The species successions in Pääjärvi produce changes in the pigment content of algae similar to those reported from polar and high alpine lakes, confirming that change in pigmentation is an important mechanism in light adaptation, whether at community or individual level. Algal pigment content was particularly high at the end of the long period of winter ice cover, indicating a degree of adaptation to the prolonged low‐light conditions, which produced the extremely high photosynthetic capacities measured at this time. However, phytoplankton production at any irradiance was primarily determined by biomass.